Aircraft batteries are, of necessity, as compact as possible due to space constraints, particularly aboard private aircraft. In cold weather, batteries need to be heated for optimal performance and, in severe cold, for any performance at all. The lead-acid batteries installed in aircraft lose a high percentage of their cranking capacity in cold weather. At -40 degrees F., approximately 10 percent of the battery capacity is available for starting. At low temperatures, the battery is unable to be recharged but rather takes on a "surface charge" which causes a battery voltage increase, shutting down the aircraft charging system. Multi-engine aircraft are often able to start one engine but then are unable to start the second one, even though the generator is "on line". A partially charged battery is subject to freezing.
The aircraft battery is more lightly built than automobile batteries and is sized to the minimum needs of the aircraft. This is done to save weight. In cold weather, the aircraft is short on battery power. Since the cold battery does not recharge readily, the aircraft voltage regulator is set to put out more voltage in cold weather. This helps until the engine compartment warms up, then the charging system "backs off". After engine start, if an aircraft loses its generating system, a pilot can normally expect about an hour of operation of critical flight systems from the battery at normal operating temperatures. In cold weather, the operational time may be reduced to a few minutes due to the surface charging of the battery.
The aircraft battery needs preheating in cold weather, but this is difficult to accomplish because of its fragile nature and the places in which it is installed. The battery is normally in a sealed box with drains and vents. Sometimes this box is made of plastic which melts when heated. Burning light bulbs have been placed proximate the battery to preheat it. This provides only localized heat and poses the danger of fire. The use of hot air blowing preheaters is only marginally effective and requires some dismantling and assembling of the aircraft. Automotive-type battery warmers are designed for larger batteries and have poor temperature regulation.